Jordi Boggiano put together a list of stats on PHP version usage from Packagist:
A few observations: 5.3 lost 5% which is good but now I guess we are on a long tail decline of Ubuntu 12.04 machines, plus a lot of libs still test against it on Travis which might bias the numbers a bit. 5.5 is still the major platform with a stable 50%, and 5.6 adoption gained 15% that were lost by 5.4. We also see 7.0 appearing slowly, mostly I assume from travis builds again.
Source: Laravel News